Miniature ballscrews typically have nominal diameters ranging from 3 to 16 mm. They are selected for their smooth operation with constant idling torque, rather than for their high load capacity.

These ballscrews are mostly used in devices and instruments that transport small objects, such as semiconductor devices or microscopic items. Because these instruments are sensitive to any motion resistance, friction of all their mechanical components must be low and constant. One of the reasons engineers choose miniature ballscrews, particularly those with diameters from 3 to 5 mm, is because they can deliver the required smooth motion.

Along with smoothness and lead accuracy, engineers should also consider the type of thread, any effects from the application’s required linear speed, style of ballscrew nut, and determine the need for backlash.

Accuracy. Ballscrew accuracy is a combined function of the controller-motor- ballscrew-bearing-encoder system. There are several grades, (see the box, “Grading accuracy.”)

The accuracy of the lead length (the distance between threads on a ballscrew) is one of the main components of ballscrew accuracy. As a general rule, the smaller the ballscrew, the higher the lead accuracy necessary to ensure proper function. The optimum mechanical drive is often a ballscrew with lead fine enough to provide the desired resolution but coarse enough to provide sufficient thrust. Miniature ballscrews typically have a high lead accuracy, from 0.5 to 5 mm. If preloaded linear bearings are used as guideway elements, the optimum lead for maximum repeatability ranges between 2 and 4 mm.

A surface scanning instrument, for example, may use a ballscrew with accuracy class zero (maximum lead error of 4 mm per 300 mm thread length), even though it actuates an auxiliary operation rather than the main measuring function of the instrument.

Small leads help achieve fine positioning. However, the small ball diameters used with these leads have less load capacity and lower allowable preload, which reduces overall stiffness. This is because preload can eliminate backlash and increase overall ballscrew stiffness.

In an internal ball deflector nut, the deflector smoothly returns the balls to the preceding thread, for continuous motion.

Engineers should choose the lead length based on the required maximum velocity. The two main constraints to consider are nut speed (based on the ballscrew nut style) and critical screw speed. Manufacturer’s publish the maximum nut speed in their specifications. However, you will find that a nut with internal recirculation often has a higher value than one with an external tube. You can calculate the critical speed of the spinning thread from the supportbearing method used (fixed-fixed, fixedsupported, and so on), the nominal diameter, and the overall length.

Load. As mentioned earlier, ballscrews with diameters of less than 8 mm are usually chosen for their smooth operation or to fit tight space constraints. Ballscrews from 8 to 16 mm in diameter are selected according to load carrying capacities and lead. Engineers can use the formulas for static and dynamic-load capacities available in manufacturers catalogs to select the proper miniature ballscrew size.

Threads. Threads on the 3 to 8 mm diameter ballscrews are usually precision ground. As the ball sizes get smaller, the sensitivity of the miniature ballscrew to machining tolerances increases. Ball diameters, restricted by lead and diameter of the screw, are typically equal to or less than 1 mm. Thus, only precision grinding can ensure proper operation of the ball circuit.

The threads of ballscrews over 8 mm in diameter may be ground or rolled.

Linear speeds. An important design guideline, often overlooked, is that high linear speeds (about 2,000 rpm) affect the required minimum thread accuracy. High linear velocity often involves high accelerations, usually more than 1,500 rad/sec2. With such accelerations, balls may skid rather than roll. This will cause the lubrication film to fail, which will lead to excess heat generation and subsequent failure of the ballscrew. Preloading the nut and special lubrication can avoid this condition.

In turn, though, the need for a preloaded nut might call for a screw of higher accuracy grade than otherwise required.

At high speeds, vibration is a concern. Thus, the straightness of the screw and concentricity of the bearing points and ballnut races become important factors. Any of which may point to the need for a ballscrew with much better lead accuracy than initially specified.

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